Identification of a new amino acid residue capable of modulating agonist efficacy at the homomeric nicotinic acetylcholine receptor, alpha 7

Neuronal nicotinic receptors (nAChRs) have been implicated in pathology ass ociated with neurological diseases and aberrant cognitive states such as ad diction and schizophrenia. The design of subtype-specific cholinergic drugs is dependent on identification of key amino acids that play a significant role in determining subunit-specific agonist efficacy. 1,1-Dimethyl-4-pheny lpiperazinium (DMPP) and a series of piperazium (PIP)-derived cholinergic a gonists (1,1 dimethyl-4-acetylpiperizinium iodide, EthylPIP, PropylPIP, and ButylPIP) were used to identify a site (position 84) in homomeric neuronal nAChRs, which is a partial determinant of pharmacological specificity. In contrast to absolutely conserved amino acids within the nicotinic superfami ly, the amino acid in position 84 can be polar or nonpolar. The addition of one methylene to PropylPIP to form ButylPIP eliminated channel activation of but not binding to the chick alpha 7 homomeric nAChR (leucine in positio n 84). In rat alpha 7 (glutamine in position 84), ButylPIP was an agonist. 1,l-Dimethyl-4-phenylpiperazinium, a structural analog of ButylPIP, activat es the rat alpha 7 but is a weak partial agonist of the chick alpha 7. Muta tion of the chick alpha 7 (L84Q) restored activation by ButylPIP, and the c orresponding mutation in rat alpha 7 (Q84L) abolished activation by ButylPI P. These mutations had moderate effects on the apparent affinity for acetyl choline, increasing its affinity for chick alpha 7 and decreasing it for ra t alpha 7. Thus, the amino acid in position 84 (a residue on the periphery of the highly conserved loop A of the cys-loop superfamily of receptors) ca n potentially be exploited to produce subtype-specific drugs and can provid e insights into the structure of the binding domain.